Catheters are medical devices in the form of hollow flexible tubes for insertion into a part of the body usually to permit the passage of fluids or keep open a passageway. A catheter is normally accompanied with accessory components such as a control handle, catheter tips, surgical tools, etc., depending upon the application (and thus as a whole may be referred to, more properly, as a catheter system). In minimally invasive medical procedures, catheters are often used to deliver therapy in such a way that requires a respective catheter tip to be in contact with the tissue being treated. Radio frequency ablation (RFA) is one example of such a procedure, wherein the therapy is carried out with an ablation catheter having a tip that delivers high frequency alternating current so as to cause heating of the tissue.
While some RFA procedures involve placing the ablation tip inside the tissue to be treated, such as in the treatment of tumors, others involve only touching the ablation tip directly against the tissue surface, such as in the treatment of cardiac arrhythmias. In the latter type of procedure, where the tip only touches the tissue surface, without penetrating the tissue, the success of the procedure is partly dependent on how forcefully the ablation tip contacts the tissue surface. If the tip is not in good contact with the tissue surface, the heating therapy will be diminished. If the tip is firmly contacting the tissue surface with some force, as opposed to just lightly contacting the surface, the heating therapy will be more effective.
In the case of an RFA procedure in cardiac electrophysiology (EP), the goal is to have the RFA heat the tissue to the point of causing lesions that will block certain electrical pathways in the heart tissue that are contributing to the arrhythmia. Consequently, the degree of contact of the ablation tip against the tissue is highly important in the success of the therapy. To effectively block the electrical signal the lesions should have some depth within the tissue, as opposed to just being formed in a thin layer of the tissue surface. The depth of the lesion depends on both the contact force and the ablation power supplied to the tip. If lesions of sufficient depth and area are not being formed, because of insufficient contact and/or power, the RFA procedure will tend to be much longer and there will be a higher probability that the procedure will not be successful in stopping the arrhythmias, such that a follow-up procedure will be needed. Conversely, if there is too much force and/or too much power there are potential risks including penetration of the tissue wall, esophageal injury, cardiac tamponade or perforations from steam pops (particularly during irrigated ablation procedures at high power) (this is noted in further detail in a presentation by Y. Yang, entitled “Atrial Fibrillation Ablation, The emerging role of stereotaxis”, University of California Davis Medical Center, Department of Internal Medicine, Division of Cardiovascular Medicine, 2011, http://wvvw.ucdmc.ucdavis.edu/internalmedicine/cardio/pdf/atrial%20fibrillation%20ablation%202011.pdf). Thus, successful cardiac RFA therapy seeks to form effective lesions while still minimizing the risk of complications. Both are dependent upon controlling the degree of contact of the ablation tip against the tissue.
RFA procedures are routinely performed under image guidance (usually fluoroscopy or ultrasound). While image guidance systems and techniques can provide visualization of the catheter tip, and sometimes localization of the tip within some coordinate space, the challenge is often in relating that tip information to the actual location of the anatomy of interest. Sometimes this might be accomplished by using optimal imaging planes that clearly show both the anatomy and the device, although this can be difficult in a complex anatomy such as the heart. In the case of the heart, this is further complicated by the heart beating motion, patient breathing motion and catheter motion. Other techniques involve the use of pre-acquired volumetric imaging data or 3D models of the anatomy superimposed with the real-time imaging, but these may also have inaccuracies due to registration errors stemming from the local motions, as well as from more global patient shifts. Thus, using imaging techniques alone, it can be very difficult to definitively judge whether an ablation tip is in good or appropriate contact with the tissue surface or not.
In current practice there are several means of assessing whether a good ablation is being achieved at a certain instance. While the user has some feel of the resistance as the catheter is navigated towards the target anatomy, once at the target there usually isn't enough sensitivity for the user to tell how good the contact is between the ablation tip and the tissue surface. Many catheter systems and methods for measuring tip contact force rely on some form of sensor built into the tip, such as fiber optic force sensors, piezoelectric strain gauges or other such devices. Some systems relay signals (electric, optical or fluid-based) back to the catheter's hand control, translating that signal into a corresponding force in attempt to give a truer tactile feedback to the user. Other systems provide quantitative measures which can be displayed to the user to help gauge the force of the tip contact.
In a cardiac EP system the catheters also have electrodes which measure the electrical impedance of the heart tissue, as part of a mapping function for planning where to ablate and also for checking for changes during the RFA procedure. The lesion formation is affected by important relationships between the tissue impedance and the power delivered to the ablation tip. The tissue impedance measurement also can give some indication of the tip contact, as the impedance will be increased when the tip is in good contact with the tissue.
In terms of actual catheter tip contact force, various studies use different levels to characterize their results. Although derived from cardiac perspectives, some general guidelines are offered by the Yang presentation noted above and an article by K. Yokoyama, H. Nakagawa, D. C. Shah, et al., entitled “Novel Contact Force Sensor Incorporated in Irrigated Radiofrequency Ablation Catheter Predicts Lesion Size and Incidence of Steam Pop and Thrombus”, Circulation: Arrhythmia and Electrophysiology, 2008, pp. 354-362, Vol. 1, American Heart Association. An example of characterizing force levels may be described as follows:
<10-15 g low contact force, ablation ineffective
20-25 g medium contact force
>40-60 g high contact force
>100 g at risk of perforation of cardiac tissue
With respect to cardiac RFA applications, however, the tip contact force must be considered along with power, impedance and temperature when attempting to achieve optimal lesions. Duration of a particular ablation is another important factor in lesion formation, with various techniques being used, ranging from short intermittent ablations at individual points to long sustained ablations where the tip may be dragged over an area for a minute or more. In this aspect, the consistency of the tip contact force is another important factor.
Several companies currently have force-sensing ablation catheters in clinical trials, including the TactiCath from Endosense and the Thermocool Smarttouch from Biosense Webster. The TactiCath uses a fiber-optic based force sensor. The SmartTouch uses magnetic signal based force sensors. Neither of these catheters are compatible with magnetic resonance imaging (MRI), but their use in various studies and clinical trials do offer evidence of the clinical benefits of having a force sensing capability (this is detailed further in the Yokoyama article noted above and an article by B. Schmidt, et al., entitled “TOCCATA Multi-Center Clinical Study: Irrigated RF Ablation Catheter with an Integrated Contact Force Sensor—Long-term Results”, Heart Rhythm 2010, 2010, PO2-59, Heart Rhythm Society). The ability to perform, for example, RFA procedures, under MRI would be desirable for enhanced image guidance since, among other reasons, MRI provides superior soft tissue contrast in images and an ability to track devices in 3D space. Work is being done on MRI-compatible force-sensing catheters at King's College London, which use fiber-optic based force sensing (this is described in more detail in an article by P. Polygerinos, A. Ataollahi, T Schaeffter, et al., entitled “MRI-Compatible Intensity-Modulated Force Sensor for Cardiac Catheterization Procedures”, IEEE Transactions on Biomedical Engineering, 2011 March, 58(3), pp 721-726). A completely different force-sensing approach is offered in the Stereotaxis Remote Magnetic Navigation System (detailed in the Yang presentation noted above) which claims “soft and consistent tissue contact” as one of its benefits. While using magnetics to control the catheter movement, this system is not MRI-compatible and instead is usually integrated with a fluoroscopy system.
Work is also ongoing in the area of MRI-compatible ablation catheters (for example, MRI Interventions (formerly SurgiVision) in collaboration with Siemens; Philips; Imricor in collaboration with GE; and at various research institutions, although all of these efforts are still in pre-clinical phases. None of these efforts publicly mention force-sensing capability as part of their MRI-compatible catheter work. The challenges of making a catheter MRI-compatible are compounded by the multiple functions required (delivery of ablation energy, measurement of impedance, measurement of tracking coil signals, etc.). While a force-sensing tip would offer clinical benefits, the addition of that capability in an MRI-compatible catheter would further complicate the design.